The present invention relates to spinal implant devices and associated techniques for promoting fusion of two or more vertebrae.
It is known that when an intervertebral disc degenerates or is damaged, there is often a compression of the disc and a reduction in the normal intervertebral height. Typically, this condition results in abnormal motions that become a source of pain.
In order to treat a pathology of this type, the disc is often stabilized to eliminate the abnormal motions caused by disc disorders or injuries. Generally, one approach is to prevent articulation between the two vertebrae situated on each side of the damaged disc by bone fusion. This fusion fixes the vertebrae to each other, eliminating the relative mobility causing the pain.
Various spinal implants to promote fusion between adjacent vertebrae have been proposed. It has been proposed to interconnect the two vertebrae by a kind of rigid U-shaped stirrup which restores the discal height with a bone graft material disposed inside the stirrup. However, one drawback of this proposal is its diminishing effectiveness over a period of time.
An spinal cage is also known which consists of a cylindrical member provided with a series of openings and provided with anchoring points. This implant is placed in a recess formed in the intervertebral disc and penetrates the opposite cortical plates of the two vertebrae which were previously hollowed out to receive the implant. This penetration forms openings in the sub-chondral plates to place spongy bone of the vertebrae in contact with bone graft material placed inside the implant, facilitating bone fusion. U.S. Pat. No. 5,015,247 provides one example of this approach.
FR 2,736,538 discloses an intersomatic cage formed by a body having inclined upper and lower surfaces and parallel lateral walls, provided at their end portions with anti-backward movement teeth. In such a structure the central and end surfaces are in the same plane so that the major part of the bearing surface is provided by the central part, that is in contact with the spongy bone, of which the strength is lower than the strength of cortical plates. Hence such structure is not entirely satisfying.
Proper performance of a spinal implant of this type requires balancing the need to promote fusion between the spongy bone and the need to form a reliable load bearing relationship with the stronger cortical bone. As a result, the spinal cage must be neither engaged too far into the openings provided in the cortical plates to provide a sufficiently dense load bearing surface, nor insufficiently inserted, in which case the bone fusion between the two vertebrae would be adversely affected by a poor anchorage. Thus, there is a demand for devices and techniques that facilitate attaining the proper balance between fusion and load support.
The present invention meets this demand and provides other significant benefits and advantages.
The present invention relates to spinal fusion implants and techniques. Various aspects of the present invention are novel, nonobvious, and provide various advantages. While the actual nature of the invention covered herein can only be determined with reference to the claims appended hereto, certain forms which are characteristic of the preferred embodiments disclosed herein are described briefly as follows.
According to one form of the invention, the spinal implant comprises a body having a central part arranged to allow arthrodesis and at least one terminal part for bearing against the cortical bone of the vertebral end plates. The central part is adapted to penetrate the vertebral end plates, transversely projecting from the terminal bearing part. Thus the invention achieves a separation between the end parts constituting the load bearers, and the intermediate part of the implant which permits fusion. In addition, the central part may include at least one cavity for receiving a bone graft material.
In another form of the present invention, an implant for insertion between a first vertebra having a first cortical bone plate and a second vertebra having a second cortical bone plate includes two terminal parts. The first terminal part defines a first bearing surface to bear against the first cortical bone plate and a second bearing surface opposite the first surface to bear against the second cortical bone plate. The second terminal part opposes the first terminal part and defines a third bearing surface to bear against the first cortical bone plate and a fourth bearing surface opposite the third surface to bear against the second cortical bone plate. The implant has an elongated central part defining an upper projection extending past the first and third surfaces, and a lower projection extending past the second and fourth surfaces. These projections correspondingly pass through openings in the first and second cortical bone plates when the first and third surfaces bear against the first cortical bone plate and the second and fourth surfaces bear against the second cortical bone plate. The terminal parts are dimensioned to facilitate restoration of the natural geometry of the intervertebral space (lordosis, cyphosis, and parallel discs). Thus, the first and second surfaces may be separated by a first distance, and the third and fourth surface may be separated by a second distance greater than the first distance to accommodate a natural curvature of the spine.
In a further form of the present invention, an implant with two terminal parts also has an elongated central part that includes a pair of longitudinal walls defining a cavity. The walls define a first edge projecting past the first and third surfaces and a second edge projecting past the second and fourth surfaces. The first and second edges correspondingly penetrate the first and second cortical bone plates when the first and third surfaces bear against the first cortical bone plate and the second and fourth surfaces bear against the second cortical bone plate.
According to another form, the bearing surfaces of the terminal end parts are defined by flanges extending from opposing ends of the implant along its longitudinal axis. Preferably, the bearing surfaces are generally flat for bearing against the cortical bone of the vertebral end plates of the two adjacent vertebrae. It is also preferred that openings be cut into the cortical plates in their central regions corresponding to the length of a central part of the implant along the longitudinal axis and leaving a region of the cortical bone plates around the periphery of the openings. The length of the remaining peripheral plate corresponds to the length of the bearing surfaces along the longitudinal axis. When the implant is placed in position, the edges of the walls of the central part engage the openings cut in the cortical plates and consequently do not substantially bear against the remaining peripheral portion of the plates. A cavity may be defined by the central part that holds bone graft material in contact with the spongy bone of the two vertebrae. In contrast, the bearing surfaces of the flanges are disposed adjacent the edges of the openings of the cortical plates and bear against the remaining portions of the plates to establish a strong load bearing relationship. Thus, both bone fusion and support are distinctly accommodated by different parts of the implant structure, which permits obtaining a satisfactory support of the vertebral bodies on the implant and an excellent arthrodesis.
Yet another form of the present invention includes a cutting tool accessory to prepare the cortical plates of two adjacent vertebrae for insertion of an implant. This tool comprises a proximal handle connected to an elongate shaft configured to rotate about a longitudinal axis of the tool. The tool also includes a first non-cutting portion with the shaft extending therethrough and being configured to rotate relative thereto. A cutting portion is fixed to the shaft to rotate therewith and is positioned distal to the first non-cutting portion. The cutting portion includes a first pair of generally parallel opposing faces and a second pair of opposing faces each extending between the first pair of faces. The second pair of faces each define a number of cutting teeth. A second non-cutting portion is fixed to the cutting portion that includes a distal head. The first non-cutting portion, the cutting portion, and the second non-cutting portion have a rotatably selectable alignment that presents a generally constant height corresponding to the intersomatic space defined between the cortical bone plates to facilitate insertion therein. Once inserted, the cutting portion may be rotated out of this alignment to cut a first opening in the first cortical bone plate and a second opening into the second cortical bone plate. The cutting portion and to non-cutting portions may be arranged to provide uniform, symmetrical cutting of these openings with a predetermined length corresponding to the dimensions of a given implant device.
In an additional form, a technique of spinal fixation includes cutting adjacent vertebrae and inserting an implant therebetween to promote fusion and provide suitable support. The implant may be inserted by anterior or posterior surgical approaches. The cutting may be performed by the cutting tool of the present invention and may include initially inserting the tool so that a first pair of faces are in contact with a respective one of the first and second cortical bone plates, turning a handle to rotate the cutting portion to remove cortical bone with cutting teeth defined by a second pair of faces, and withdrawing the tool. The tool may be used to form openings readily positioned in the central region of the adjacent vertebrae that leaves cortical bone plate about the openings. The insertion of the implant may include positioning the implant of the present invention between the first and second vertebrae and turning the implant about one quarter of a turn.
It is envisioned that the implants of the present invention may be used with other tools and procedures, that the tools of the present invention may be used with other implants and procedures, and that the procedures of the present invention may be used with other implants and tools as would occur to those skilled in the art without departing from the spirit of the present invention.
Accordingly, one object of the invention is to provide a spinal implant arranged to facilitate proper fusion of two or more vertebrae. The implant may be arranged to have at least one structural part to promote bone fusion through contact with spongy vertebral bone and one or more other structural parts adapted for contact with cortical bone to provide load support.
Another object is to provide a tool accessory to facilitate implantation of devices suitable to promote fusion.
Still another object is to provide a technique for preparing a site between two vertebrae to be fused and inserting an implant.
Other objects, features, benefits, forms, aspects, and advantages of the present invention will become apparent from the description and drawings herein.